Electric heating element



Patented Feb. 16, 1943 ELECTRIC nmrmc murmur m: Chambers Chaston, Wembley, Mlddlesex,

England No Application December 27, 1940, Se-

iisaalgNo. 372,015. In Great Britain December 20,

4 Claims.

This invention relates to electric resistance heating elements, such as furnace windings (hereinafter termed heating resistors) constructed of platinum alloys having improved characteristics.

It has hitherto been usual to employ pure platinum or platinum alloys containing rhodium for the purpose of forming the heating resistors of electric resistance furnaces for operation at temperatures up to about 16500. For these, and similar, applications the rhodium-platinum alloys have the advantage that their temperaturecoeflicients of resistance are appreciably lower than that of pure platinum. The values for the temperature coeiiicient of resistance of 10% rhodiumplatinum alloy and of pure platinum are 0.0013 and 0.0039 per degree centigrade respectively over the range from to 100 centigrade. As a result, the resistance of a furnace wound with rhodium-platinum alloy does not increase to such an extent as that of one wound with pure platinum as the temperature is raised; the furnace wound with 10% rhodium-platinum alloy is easier to control; it requires less attention during the initial stages of heating; and it does not require to have so large an external resistance connected.

in series with it for satisfactory operation.

. urements of the electrical resistance of the wire I have now found that the alloys of-platinum with from 1 to 20% of ruthenium by weight have values of temperature coefllcient of resistance which are very much smaller than those of the corresponding rhodium-platinum alloys and have characteristics which make them very well adaptnium-platinum alloys are unsuitable for operation at high temperatures in view of the volatility of the oxides of ruthenium. I have now found that, with proper precautions, it is possible to prepare with ease wires of the ruthenium-platinum alloys containing up to 15% of ruthenium and, with. care, to produce and work alloys containing up to 20% of ruthenium. Furthermore, I have found that no appreciable change occurs in the composition of the alloys when they are made into the form of heating resistors such as furnace at 0 centigrade and centigrade revealed that the average temperature coefficient of resistance between these two temperatures was only 0.00089, which is less than of that of the 10% rhodium-platinum alloy. Alloys containing greater amounts of ruthenium were found to have still lower values of temperature coefilcient of resistance, as shown in the following table:

Alloy: Temperature coeflicient of resistance per 0. 10% ruthenium, balance platinum 0.00046 14% ruthenium, balance platinum 0.00036 The value of the 5% ruthenium-platinum alloy for the purpose of forming a heating resistor was shown by the results of tests on a tubular furnace in which the heating resistor was composed of a length of 5% ruthenium-platinum alloy wire, 0L020 inch in diameter, wound over a refractory tube 1.0 inch in diameter.

The resistance of the furnace was found to be 5.63 ohms at 18 C., and increased to only 9.7 ohms when the temperature of the furnace was raised to 1500 C. If the furnace had been wound with 10% rhodium-platinum the resistance at 1500 C. would have been about 15.7 ohms, and if wound with pure platinum 23.15 ohms.

The furnace was operated continuously at 1500 C. and at the end of 144 hours, when the test was stopped, it was in excellent condition and the resistance of the winding had increased only to 10.9 ohms. This change of resistance is no greater than occurs with windings of pure platinum or rhodium-platinum and is probably largely due to volatilisation of the winding. Half'the increase occurred during the first 24 hours of operation, and it is evident that no appreciable loss of ruthenium occurred by oxidation.

I claim: V

1. An electric furnace resistor having a low temperature coeflicient of resistance comprising a platinum-rutheniwn wire esntaining from 1 to 20% of ruthenium,

2. An electric furnace resiswr having a "low temperature coefficient of resistance comprising a, pla.tinum=ruthenium wire containing 5% ruthe nium.

3. An electric iurnaca resistor having a, low temperature coefiiclent of resistsmce comprising a 331 was platinum ruthenium wire mum An electric yesistar having a iaw temperature coefiicient of resistance comprising a plenum-ruthenium wire cont asining' 14% ruthemum. 

